In recent years, the miniaturization of circuit boards and the integration of semiconductor elements have advanced rapidly and further improvements in the heat dissipation characteristics of circuit boards, particularly ceramic circuit boards using ceramics as a substrate are desired. Ceramics such as silicon carbide (SiC) to which beryllia (BeO) has been added, aluminum nitride (AlN), silicon nitride (Si3N4), etc. have attracted attention as ceramic circuit boards having excellent heat dissipation properties.
When using the above ceramic substrates as circuit boards or substrates for packages, heat emitted from electrical and electronic components such as semiconductor elements and transmitted to ceramic substrates is discharged to the outside via a heat dissipating component called a heat sink provided on the back side of the circuit board, which prevents the occurrence of malfunctions due to temperature rises in the semiconductor components and ensures the performance characteristics of the circuit board.
Copper is known in typical heat sinks, but when applied to a ceramic circuit board, there is the problem of, due to the difference in the thermal expansion coefficients of copper and the ceramic circuit board, cracks or fractures occurring in the ceramic substrate when heated or exposed to heat cycles in which the semiconductor components operate and halt, or cracks arising in soldering parts coupling the ceramic substrate and the heat sink.
For this reason, in fields that demand particularly high reliability, Mo/W, the thermal expansion coefficient of which differs little from that of ceramic substrates, is used as a heat sink. However, the specific weights of each of the metals in Mo/W are high, causing heat sinks or ceramic circuit boards to which they are bonded to be heavier, so Mo/W is not preferred for uses in which the lightening of heat dissipating components is desired, for example, for installation on moving devices such as automobiles or train cars. Furthermore, Mo and W also have the flaw of being rare and expensive.
Due to the abovementioned circumstances, metal-ceramic composites known as MMCs (Metal Matrix Composites) in which copper, aluminum or alloys thereof are reinforced with inorganic particles or fibers, have gathered attention in recent years. (Patent Documents 3 and 4)
MMCs generally are composites in which a preform is formed by molding inorganic particles or fibers which are reinforcing materials ahead of time, and impregnating a metal or alloy between the reinforcing materials in the preform. Alumina, silicon carbide, aluminum nitride, silicon nitride, silica, carbon, etc. are used in reinforcing materials. (Patent Document 1)
When attempting to raise the thermal conductivity of a metal-ceramic composite, it is necessary to select substances having high thermal conductivity as the reinforcing materials and the metal or alloy to be impregnated. Additionally, the wettability, interface reaction, etc. of the reinforcing materials and the metal or alloy have effects on the thermal conductivity and strength of the obtained metal-ceramic composite. (Patent Document 2)
For application to the above uses, the use of metals having aluminum as a main component in a preform having silicon carbide as a main component in the reinforcing materials as a combination obtaining a metal-ceramic composite provided with light weight, high thermal conductivity and moreover, a low thermal expansion coefficient of a similar degree to that of various ceramic substrates, is gathering attention. (Patent Document 4)
[Patent Document 1] JP H05-238804 A
[Patent Document 2] JP S59-199587 A
[Patent Document 3] JP H10-219368 A
[Patent Document 4] JP 2000-169267 A